The iron-overlay-behavior tries to prevent the user from tabbing outside
of the overlay by recording the first and last child elements which
appear to be focusable. It intercepts [Tab] and [Shift+Tab] keystrokes
to wrap the tab-order of its controls. However, the way it determines
the first and last such element is 1) not composable with child elements
that are Polymer components and 2) not composable with dynamically
changing children.
Because gr-reply-dialog is both 1) built with Polymer components and 2)
partially dynamically generated, the tab order is broken when the
overlay behavior tries to intercept and constrain it.
The workaround here is to remove the iron-overlay-behavior's guesswork
and manually set the first and last focusable inputs with knowledge of
what those elements would be in gr-reply-dialog.
gr-overlay's #open method now returns a promise that resolves when the
overlay is actually displayed.
Bug: Issue 4165
Change-Id: Ib1cdea4c5e99f821db699c89743240caab751f03
165d5d33b5